December 16, 2010 (estimated); the 10th day of Muharram (New Year) in the Islamic calendar, commemorated as a day of mourning for the martyrdom of Husayn ibn Ali, the grandson of the Islamic Prophet Muhammad.
Note that in the Muslim calendar, a holiday begins on the sunset of the previous day, so observing Muslims will celebrate Ashura on the sunset of Wednesday, the 15th of December.
The Day of Ashura (عاشوراء (ʻĀshūrā’, Ashura, Ashoura, and other spellings) is on the 10th day of Muharram in the Islamic calendar and marks the climax of the Remembrance of Muharram.
It is commemorated by the Muslims as a day of mourning for the martyrdom of Husayn ibn Ali, the grandson of the Islamic Prophet Muhammad at the Battle of Karbala. Sunni Muslims believe that Moses fasted on that day to express gratitude to God for liberation of Israelites from Egypt.
-information taken from http://en.wikipedia.org/wiki/Day_of_Ashura